This application is related to Japanese application No. HEI 10-281433 filed on Oct. 2, 1998, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which Is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a process for producing a positive electrode active material for a nonaqueous secondary battery and also relates to a nonagueous secondary battery. More particularly, the invention relates to a process for producing a positive electrode active material for a nonaqueous secondary battery, the active material being represented by the formula, LiNi1xe2x88x92xMxO2 wherein 0xe2x89xa6X less than 0.5 and M is at least one element selected from transition metals, group 3B elements, group 4B elements and group 5B elements, and a nonaqueous secondary battery with use of a positive electrode including at least one kind of positive electrode active material produced by the process.
2. Description of Related Art
With reduction of the size, the weight and the power consumption of electronic equipment and like devices, progress has been made in research and development of secondary batteries using alkali metals such as lithium which are lightweight and able to generate high voltage. Now lithium ion secondary batteries using carbon or graphite for a negative electrode active material and lithium cobalt oxide for the positive electrode active material are in practical use.
In the case where lithium cobalt oxide is used for the positive electrode active material, there is a problem that raw material costs are high because cobalt resources are limited. Accordingly, since John Banister Goodenough et al proposed lithium nickel oxide (LiNiO2) using nickel whose resources are more abundant than the cobalt resources and which is available at a lower cost (see Japanese Patent Publication No. SHO 63(1988)-59507), active research have been made on positive electrode active materials containing lithium nickel oxide.
The following processes have been reported for production of lithium nickel oxide:
(1) A process of calcining anhydrous lithium hydroxide together with a metal nickel in an atmosphere of oxygen (see J.Am.Cher.Soc., 76, 1499(1954));
(2) A process of mixing LiOH.H2O and NiO, calcining the mixture at 600xc2x0 C. in an atmosphere of air, pulverizing it, and then calcining it again at 600xc2x0 C. to 800xc2x0 C. (see Japanese Unexamined Patent Publication No. HEI 2(1990)-40861);
(3) A process of calcining a mixture of a compound as a lithium source and a compound as a nickel source at 600xc2x0 C. to 800xc2x0 C. (preferably at 800xc2x0 C. for six hours twice) (see Japanese Unexamined Patent Publication No. HEI 4(1992)-181660);
(4) A process of mixing lithium peroxide (Li2 O2) and nickel oxide (NiO), allowing the mixture to react at a temperature not higher than 750xc2x0 C. and then quenching the mixture from that temperature (see Japanese Unexamined Patent Publication No. HEI 5(1993)-205741); and
(5) A process of mixing lithium nitrate with at least one of nickel hydroxide and nickel oxyhydroxide and calcining the mixture at 500 to 1,000xc2x0 C. (see Japanese Unexamined Patent Publication No. HEI 5(1993)-251079).
In these processes, a lithium source compound and a nickel source compound are mixed in the state of solid and then calcined.
As another process, there is proposed a process of dissolving nickel acetate and lithium acetate in ethylene glycol with heating, further heating the mixture to set It up, thermally treating the resulting substance at 400xc2x0 C. in air, pulverizing it, calcining it in an oxidizing flow at 700xc2x0 C. and then calcining it at 800xc2x0 C. in an oxidizing flow (see Japanese Unexamined Patent Publication No. HEI 6(1994)-203834).
Further, known are the following processes for uniformly mixing a lithium compound and a nickel compound to facilitate the production of the positive electrode active material:
(a) A process of equimolarly mixing an aqueous solution of lithium hydroxide of 4.5 mol/L and an aqueous solution of nickel nitrate of 1.0 mol/L at 60xc2x0 C., stirring the resulting mixture, drying it under reduced pressure, pulverizing the resulting matter, preliminarily calcining the pulverized matter at 300xc2x0 C. and thereafter subjected to a major calcination at 800xc2x0 C. (see Chemistry Express, 6, 161(1991));
(b) A process of obtaining LiCoxNi1xe2x88x92xO2 (0xe2x89xa6Xxe2x89xa60.5)by dissolving a nickel salt. a lithium salt and an optional cobalt in a solvent and wet-mixing the resulting mixture, followed by calcination (see Japanese Unexamined Patent Publication No. HEI 5(1993)-325966);
(c) A process of mixing a water-soluble nickel salt and a water-soluble lithium salt in an aqueous solution, solidifying by drying the resulting mixture and calcining the resulting cake-like matter at 600xc2x0 C. to 800xc2x0 C.;
(d) A process of kneading by stirring a powdery nickel compound slightly soluble or insoluble in water with an aqueous solution of a water-soluble lithium salt, solidifying by drying the resulting mixture and calcining the resulting cake-like matter at 600xc2x0 C. to 800xc2x0 C. (see Japanese Unexamined Patent Publication No. HEI 6(1994)-44971):
(e) A process of weighing a lithium source and a nickel source in a molar ratio of 1:1 in terms of nickel and lithium, sufficiently mixing them with a little amount of water as a dispersant, followed by drying, and calcining the resulting mixture at 650xc2x0 C. in an atmosphere (see Japanese Unexamined Patent Publication No. HEI 6(1994)-96768);
(f) A process of dispersing a nickel compound in a solution of lithium nitrate, volatilizing the solvent, and calcining the resulting mixture of lithium nitrate and the nickel compound in an atmosphere containing oxygen (see Japanese Unexamined Patent Publication No. HEI 7(1995)-307165);
(g) A process of weighing a powdery nickel oxide and a lithium compound, i.e., either lithium hydroxide or lithium nitrate, in a molar ratio of 1:1 in terms of lithium and nickel, melting the lithium compound at a temperature of its melt temperature or higher and 500xc2x0 C. or lower, allowing the powdery nickel oxide to soak into the melted lithium compound, and calcining the resulting matter in the presence of oxygen or a gas containing oxygen in a high content (see Japanese Unexamined Patent Publication No. HEI 9(1997)-156931);
A process of dissolving a nickel compound and a lithium compound in an aqueous solution, adding oxalic acid to produce a nickel salt and a lithium salt as precipitates at a time, and calcining the precipitates (see Japanese Unexamined Patent Publication No. HEI 10(1998)-106564).
In the case of the above-mentioned processes (1) to (5), there is a problem that the mixture before calcination is not uniform.
In the case where ethylene glycol is used as a solvent, there are problems that reaction time period is long and that the production process is complicated.
In the case of the above-mentioned processes (a) to (c), the mixed state of nickel and lithium is somewhat improved compared with cases where they are mixed in a solid state. However, the uniformity of the mixture is still insufficient.
In the case of the above-mentioned processes (d) to (f), there Is a problem that the mixture is not uniform when water or a solvent as a dispersant is removed. Further, uniformity is insufficient even though a melted salt is mixed as in the above described (g).
Also, a positive electrode active material produced by any of the above-mentioned processes has a problem that, when it is used for producing a positive electrode, the active material deteriorates rapidly and good cycle characteristics cannot be obtained.
In the case of the above-mentioned process (h), since lithium and nickel are precipitated as composite oxalates, a slightly soluble precipitate in which lithium and nickel are uniformly mixed is obtained. However, since lithium oxalate is more soluble in water than other oxalates, the reproducibility of the mixed state is not sufficient. Accordingly, although a positive electrode containing lithium nickel oxide obtained by calcining this precipitate exhibits improved initial electrode characteristics and good cycle characteristics, there is a problem that an active material having stable characteristics cannot be obtained.
Also every time when the slightly soluble precipitate is prepared, the lithium salt as a raw material is required in an amount about several times to about ten times larger than the amount of lithium finally contained in the slightly soluble precipitate. Accordingly, there are problems that raw material costs are high and the lithium salt is wastefully used from the viewpoint of resources.
During study for solving the above problems, the inventors of the present invention have found that a precipitate comprised of oxalates containing a composite oxalate in which the ratio of Li:Ni is close to that of starting compounds and lithium and nickel are uniformly mixed and which Is slightly soluble in an alcohol is produced by a co-precipitation process wherein a lithium compound and a nickel compound as starting compounds are dissolved and then oxalic acid is added to the resulting mixture, and have achieved the present invention.
Accordingly, the present invention provides a process for producing a positive electrode active material for a nonaqueous secondary battery comprising the steps of: dissolving. in an alcohol as a medium, a starting material which contains metals necessary for giving a compound represented by the formula LiNi1xe2x88x92xMxO2, wherein 0xe2x89xa6X less than 0.5 and M is at least one metal selected from transition metals and 3B-, 4B- and 5B-group elements, as a positive electrode active material, and which is soluble in the alcohol; adding a sufficient amount of oxalic acid to the resulting solution to produce a precipitate comprised of oxalates including a composite oxalate slightly soluble in the alcohol: separating the precipitate comprised of oxalates including the composite oxalate slightly soluble In the alcohol from the solution: and calcining the precipitate thereby to obtain the positive electrode active material.
The production of the precipitate comprised of oxalates including the slightly soluble composite oxalate improves uniformity of a mixed state of the substance before calcination and facilitates control of the ratio of lithium to nickel, Li:Ni, in the precipitate comprised of oxalates containing the slightly soluble composite oxalate. Furthermore, since lithium oxalate is slightly soluble in an alcohol, it is easy to produce the precipitate comprised of oxalates including the slightly soluble composite oxalate containing lithium and nickel, from starting compounds. Thus, it has been found that the process of the present invention Is excellent also from the viewpoint of stability of manufacturing process and costs.
In another aspect, the present invention provides a nonagueous secondary battery with use of a positive electrode including a positive electrode active material produced by the above-described process.
It has been found that the above-mentioned nonaqueous secondary batter has improved initial electrode characteristics and good cycle characteristics.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples. while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.